U.S. patent number 5,425,925 [Application Number 08/214,597] was granted by the patent office on 1995-06-20 for multi-stage infectious waste treatment system.
This patent grant is currently assigned to Winfield Industries, Inc.. Invention is credited to Daniel Kline, Robert S. Meijer.
United States Patent |
5,425,925 |
Kline , et al. |
June 20, 1995 |
Multi-stage infectious waste treatment system
Abstract
A multi-stage treatment system for infectious waste includes a
shredding stage, a granulating stage, a wetting stage, a
disinfecting stage, and a dewatering stage which define a
continuous treatment flowpath for the infectious waste. A plurality
of blades shred and then granulate the waste in the shredding and
granulating stages while simultaneously mixing the waste with
disinfectant chemicals. The granulating stage insures that the
waste is granulated to a sufficiently small size to facilitate the
use of a relatively low concentration of a highly reactive
disinfectant. Chemicals are mixed to form a volatile, highly
reactive disinfectant which is then immediately injected into the
waste stream. A plurality of jets wet the waste mixture in the
wetting stage with the heated aqueous disinfectant. A flow
restriction removes excess aqueous liquid from the disinfected
waste in the dewatering stage and renders the product suitable for
landfilling.
Inventors: |
Kline; Daniel (Carlsbad,
CA), Meijer; Robert S. (San Diego, CA) |
Assignee: |
Winfield Industries, Inc. (San
Diego, CA)
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Family
ID: |
22799708 |
Appl.
No.: |
08/214,597 |
Filed: |
March 18, 1994 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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690116 |
Apr 23, 1991 |
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511275 |
Apr 19, 1990 |
5089228 |
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Current U.S.
Class: |
422/295; 422/307;
422/309 |
Current CPC
Class: |
A61L
11/00 (20130101); B02C 19/0075 (20130101); B09B
3/0075 (20130101) |
Current International
Class: |
A61L
11/00 (20060101); B09B 3/00 (20060101); A61L
002/24 () |
Field of
Search: |
;422/26,27,29,32,37,38,295,307,309,30 ;241/17,22,DIG.38 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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031516 |
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May 1989 |
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EP |
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WO90/03949 |
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Apr 1990 |
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EP |
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0364367 |
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Apr 1990 |
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EP |
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0382018 |
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Aug 1990 |
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EP |
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0383553 |
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Aug 1990 |
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EP |
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0423817 |
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Apr 1991 |
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EP |
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2512024 |
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Dec 1976 |
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DE |
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3317300 |
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Nov 1984 |
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DE |
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Other References
Medical Safe TEC, Inc., Series Twelve Five, The Ultimate in Total
Destruction and Decontamination of Infectious Waste, and Total
Infectious Waste Disposal Specifications. .
Bonnie Delaney, Smashing Success..
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Primary Examiner: McMahon; Timothy M.
Attorney, Agent or Firm: Nydegger & Associates
Parent Case Text
This file is a continuation-in-part application of prior patent
application for a "Method for Sterilizing and Disposing of
Infectious Waste," Ser. No. 690,116, filed Apr. 23, 1991, now
abandoned, which is a continuation-in-part of application Ser. No.
511,275, filed Apr. 19, 1990, now U.S. Pat. No. 5,089,228.
Claims
I claim:
1. An apparatus for treating infectious waste, comprising:
an inlet opening sized to receive an infectious waste;
a shredding stage positioned to receive the waste from the inlet
opening, said shredding stage having a plurality of opposingly
rotatable shredding blades for shredding the waste to a first,
small particle size;
a wetting stage positioned to inject a volatile liquid disinfectant
into the shredded waste downstream of said shredding stage, said
wetting stage including at least one liquid jet, said wetting stage
including a mixing means for mixing said volatile disinfectant
substantially at the time of injection of said volatile
disinfectant to prevent disassociation of said volatile
disinfectant prior to injection into the shredded waste;
a granulating stage positioned to receive the shredded waste from
said shredding stage, said granulating stage comprising at least
one stationary granulating blade and at least one rotatable
granulating blade rotatable past said stationary blade to granulate
the shredded waste to a second, smaller granular particle size and
to granulate any strips present in the shredded waste material;
a disinfecting stage positioned to receive the granulated waste
from said granulating stage, said disinfecting stage comprising a
reactor chamber sized to retain the granulated waste for a selected
residence time;
an auger conveyor positioned to transport the granulated waste from
said reactor chamber to an exit opening; and
a dewatering stage comprising a conical flow restriction in the
waste flowpath between said auger conveyor and said exit opening,
said dewatering stage having a liquid outlet in fluid communication
with said wetting stage to recycle liquid from said dewatering
stage to said wetting stage.
2. An apparatus for treating infectious waste as recited in claim
1, further comprising a granulating screen positioned downstream of
said granulating blades, said granulating screen having openings
sized to pass the second, smaller granular particle size waste and
to retain larger size particles of waste for continued granulation,
said granulating screen being positioned to be continually scraped
by said rotatable granulating blades.
3. An apparatus for treating infectious waste as recited in claim
1, further comprising a cylindrical dewatering screen housing said
auger conveyor, said dewatering screen having openings sized to
pass liquid while retaining the granulated waste, said dewatering
screen being positioned to contact the outer radius of said auger
conveyor to cause said auger conveyor to continually scrape
granulated waste from said dewatering screen.
4. An apparatus for treating infectious waste as recited in claim
1, further comprising a cyclone separator connected in fluid
communication with said wetting stage to remove heavy fines from
the recycled liquid prior to injection of the recycled liquid into
the shredded waste.
5. An apparatus for treating infectious waste, comprising:
an inlet opening sized to receive an infectious waste;
a plurality of opposingly rotatable shredding blades positioned to
receive the waste from the inlet opening;
a plurality of stationary shredding blades positioned between said
rotatable shredding blades, said stationary shredding blades being
spaced from said rotatable shredding blades by a selected distance
for shredding the waste to a selected first, small particle
size;
a mixing reservoir having a plurality of inlets for mixing a
plurality of stable constituents to form a volatile
disinfectant;
a plurality of injection jets in fluid communication with said
mixing reservoir to inject said volatile disinfectant into the
shredded waste downstream of said shredding blades, said jets being
positioned relative to said mixing reservoir to perform said
injection substantially at the time of mixing said constituents to
prevent disassociation of said volatile disinfectant prior to
injection into the shredded waste;
a plurality of rotatable granulating blades positioned to receive
the shredded waste from said shredding blades;
a plurality of stationary granulating blades, said stationary
granulating blades being positioned to be wiped by said rotatable
granulating blades to granulate the shredded waste to a selected
second, smaller granular particle size free of any strips of waste
material;
a granulating screen, said granulating screen having openings sized
to pass the second, smaller granular particle size waste and to
retain larger size particles of waste for continued granulation,
said granulating screen being positioned to be continually scraped
by said rotatable granulating blades;
a disinfecting chamber positioned to receive the granulated waste
passing through said granulating screen, said disinfecting chamber
being sized to retain the granulated waste for a selected residence
time;
an auger conveyor positioned to transport the granulated waste from
said disinfecting chamber to an exit opening;
a cylindrical dewatering screen housing said auger conveyor, said
dewatering screen having openings sized to pass liquid while
retaining the granulated waste, said dewatering screen being
positioned to contact the outer radius of said auger conveyor to
cause said auger conveyor to continually scrape granulated waste
from said dewatering screen;
a conical flow restriction in the waste flowpath between said auger
conveyor and said exit opening, said conical flow restriction
having a taper angle selected to ensure sufficient compaction of
the waste material to remove excess liquid, while preventing
clogging of said flow restriction; and
a recycle flow path in fluid communication with said dewatering
screen to recycle the excess liquid to said mixing reservoir.
6. An apparatus for treating infectious waste as recited in claim
5, wherein said rotatable granulating blades and said stationary
granulating blades have a substantially rectangular cross
section.
7. An apparatus for treating infectious waste as recited in claim
5, wherein said auger conveyor is inclined upwardly from said
disinfecting chamber toward said exit opening at an angle selected
to maximize the dewatering of the waste material while preventing
clogging of the auger conveyor.
8. An apparatus for treating infectious waste as recited in claim
7, wherein said waste material is composed mostly of relatively
soft material and said angle of inclination is between 10 degrees
and 20 degrees.
9. An apparatus for treating infectious waste as recited in claim
8, wherein said angle of inclination is approximately 15
degrees.
10. An apparatus for treating infectious waste as recited in claim
5, wherein said auger conveyor terminates slightly beyond the
entrance to said flow restriction.
11. An apparatus for treating infectious waste as recited in claim
5, wherein said waste material is composed mostly of relatively
soft material and said taper angle of said flow restriction is
between 15 degrees and 20 degrees.
12. An apparatus for treating infectious waste as recited in claim
11, wherein said taper angle is approximately 18 degrees.
Description
FIELD OF THE INVENTION
The present invention relates generally to treatment of infectious
waste. More particularly, the present invention relates to a system
which mechanically fragments and decontaminates infectious waste.
The present invention is particularly, though not exclusively,
useful for treating an infectious waste stream which includes a
variety of types of waste.
BACKGROUND OF THE INVENTION
The disposal of infectious waste from hospitals and other medical
establishments is a major problem. Indeed, the importance of proper
and effective infectious waste disposal has become of greater
concern in recent years, due to an increased awareness of health
problems such as the AIDS epidemic. In part because of the AIDS
epidemic, definitions of what constitutes "infectious waste" are
being broadened. Consequently, the volume of infectious waste which
must be disposed of is increasing. Accordingly, the need for a
system or apparatus which will accomplish the safe, efficacious,
and cost effective treatment of significant volumes of infectious
waste for disposal is growing.
One method for decontaminating infectious waste involves
incineration, wherein the waste is burned and the decontaminated
ashes are properly disposed. An alternative treatment method is to
disinfect the waste in a steam autoclave or ethylene oxide
autoclave prior to waste disposal. While effective for their
intended purposes, both incinerators and autoclaves present
ancillary problems. Incinerators, for example, are difficult and
costly to construct and are relatively expensive to maintain in an
environmentally safe manner. Autoclaves too, present additional
problems, such as odor, cost and operational complexity.
Additionally, waste which has been disinfected by autoclaving
typically requires further treatment procedures, such as
incineration, prior to final disposition of the waste in such
places as landfills.
With the above discussion in mind, alternative infectious waste
treatment systems have been proposed to disinfect the waste in
preparation for disposal. According to these proposals, a solid
infectious waste is contacted with a disinfectant solution
containing a chlorine compound to decontaminate the waste. The
decontaminated waste may then be disposed in ordinary
landfills.
Unfortunately, decontamination of waste using chlorine compounds
presents certain technical complications. First, liquid
disinfectant loses its disinfectant potency during prolonged
storage. Thus, there is a need to use liquid disinfectant that is
relatively "fresh" in order to achieve an acceptable degree of
waste decontamination. Second, it is relatively difficult to ensure
that an appropriate concentration of the disinfectant has contacted
the waste during the treatment process. It is also important,
however, to avoid applying too high a concentration of chlorine
compound to the waste, in order to avoid undesirable results, such
as corrosive effects and the release of toxic gasses. Significant
health risks are known to result from the discharge of chlorine to
the environment.
The most commonly used disinfectant is sodium hypochlorite,
typically as a one percent solution. The strength of the solution
is dictated by the necessity of achieving a desired rate of
bacteria kill in a given apparatus, resulting in a given rate of
use of the disinfectant when operating at a given rate of
throughput of waste. The use of a one percent solution results in
the discharge of a significant amount of chlorine into the
environment from the typical apparatus, either into the sewer or
absorbed into the processed waste. Because of its high reactivity,
chlorine dioxide is far more effective than sodium hypochlorite for
the treatment of infectious waste. Chlorine dioxide also typically
exists as a gas in solution, greatly enhancing the penetration of
the disinfectant into the waste material. Chlorine dioxide can, if
applied to properly granulated waste, achieve the necessary kill
rate at a concentration of only about 50 ppm, or only about 0.005
percent, or 5 one-thousandths of the necessary concentration of
sodium hypochlorite. Finally, the chlorine dioxide is far less
stable, rapidly disassociating into sodium chloride, water, and
citric acid. When taking into account the rate of use of sodium
hypochlorite in a typical process, and the required rate of use of
properly applied chlorine dioxide to achieve the same kill rate,
the sodium hypochlorite process results in the discharge to the
environment of approximately ten thousand times as much of the
treatment chemical. This means that the chlorine dioxide process
results in the discharge to the environment of an amount of
chlorine which is minuscule, compared to the amount of chlorine
discharged by the sodium hypochlorite process.
Unfortunately, chlorine dioxide is very corrosive, highly unstable,
and even explosive. It can not simply be substituted for sodium
hypochlorite in a process. It must be used in an apparatus designed
to properly mix the chemical, and designed to properly granulate
and handle the waste material to allow the use of a very low
concentration of the chemical. Therefore, sodium hypochlorite is
almost always used instead, even though it is less effective and
results in increased chlorine contamination of the environment. The
present invention recognizes that liquid precursors of chlorine
dioxide can be stored for relatively lengthy time periods without
losing their potency and can be mixed to form chlorine dioxide
immediately prior to use in a continuous process. The resulting
solution can be used in a very low concentration to decontaminate
infectious waste, if used in a system that mechanically reduces the
particle size of the waste to the appropriate size. The present
invention also recognizes the necessity for the correct interaction
of certain critical structural features in the waste processing
apparatus, to achieve the necessary intimate contact between the
low concentration of chlorine dioxide and the waste material, and
to properly handle the waste material to allow the conservative use
of the chlorine dioxide.
Accordingly, it is an object of the present invention to provide a
system for waste treatment in which chlorine dioxide is
appropriately mixed and then immediately blended with infectious
waste to decontaminate the waste, while preventing excessive
decomposition of the disinfectant, and while preventing any
explosion hazard. Another object of the present invention is to
provide a system for waste treatment which results in the reduction
of waste particle size to an appropriate size to allow effective
use of the disinfectant in a low concentration, while preventing
clogging of the waste stream and while maximizing the recycling of
the disinfectant. Finally, it is an object of the present invention
to provide a system for waste treatment which is relatively easy
and comparatively cost-effective to implement.
SUMMARY OF THE INVENTION
The present invention is a system for treating infectious waste
comprising a series of continuous treatment stages. The multi-stage
treatment system has an inlet stage at its front end which
comprises an opening for receiving the infectious waste. The waste
may be fed in any form through the opening, but in a preferred
embodiment, the opening is sized to receive a sealed plastic bag in
which the waste is packaged. The bags are fed through the opening
into the system in their entirety. In this manner, waste handlers
operating the present system need never come in direct contact with
the infectious waste. The waste bag has a primary compartment
containing the infectious waste, and it can have one or more
secondary prefilled and sealed compartments containing other
process additives in isolation from the waste, all of which are to
be introduced into the system. As will be seen below, the entire
contents of the bag are released from the bag and commingled during
operation of the treatment system.
The inlet opening leads to a fragmenting chamber positioned
therebelow which encloses the shredding, wetting, and granulating
stages of the system. The waste drops under the force of gravity
from the inlet stage down into the shredding stage which comprises
a plurality of opposingly rotating shredder blades. The shredding
blades destroy the waste bag, spilling its contents into the
fragmenting chamber. Any process additives contained in the bag
become mixed with the waste in the shredding stage. The blades also
function to break up any large frangible waste into small size
particles.
The wetting stage is positioned immediately beneath the shredding
stage to wet the small particle size shredded waste with the liquid
chlorine dioxide treatment solution, as the waste falls through the
shredding blades. The liquid disinfectant will more thoroughly mix
with the waste material as the waste passes farther through the
system. The wetting stage comprises a plurality of jets through
which the liquid disinfectant is pumped, with the jets being
positioned at the interior walls of the fragmenting chamber
immediately beneath the shredding blades. The jets are directed
radially into the chamber and are capable of producing a controlled
spray of the liquid disinfectant into the waste mixture. The liquid
disinfectant is preferably an aqueous chlorine dioxide solution
containing some gaseous chlorine dioxide, which has been formed by
the continuous mixing of liquid sodium chlorite and citric acid
immediately prior to injection through the jets. The liquid
disinfectant is also maintained at an elevated temperature
immediately prior to injection. The heated liquid is then injected,
to uniformly contact the falling waste mixture to form a hot mash.
While the present invention in its preferred embodiment makes
possible the use of chlorine dioxide as the disinfectant, the
apparatus can be used with other disinfectants without departing
from the spirit of the invention.
The granulating stage is provided beneath the wetting stage and
comprises a plurality of specially designed blades mounted on a
shaft so as to be rotatable against a plurality of stationary
blades mounted on the walls of the fragmenting chamber to form
cutting surfaces. The granulating blades rotate in a radial plane
which is substantially parallel to the flow of the mash. At the
cutting surfaces, the granulating blades break up the already small
particle size waste into yet smaller particle sizes, to insure
intimate contact between the treatment chemical and the waste
material, and to cut any fibrous material which has not been
previously fragmented by the shredding blades. The granulator
blades are designed to allow the use of a plurality of cutting
edges as the edges wear as a result of contact with hard materials.
The blades also fully mix the components of the waste mash, thereby
ensuring that the disinfectant chemicals in the liquid medium
adequately contact the waste material to achieve the necessary kill
rate with a relatively low concentration of the chemical.
A granulator is used instead of a hammer mill, because a hammer
mill will not consistently and efficiently reduce the particle size
of non-brittle materials, which constitute the majority of the
medical waste stream. When such soft materials are passed through a
hammer mill, the materials tend to pass through in crumpled form,
rather than having a reduced particle size. This results in reduced
contact between the disinfectant and the waste material.
A granulator is used at this stage instead of a shredder, because a
shredder produces long strips of material. The strips tend to clog
the shredder if used with a sizing screen, and they tend to become
folded in accordion folds, thereby reducing contact between the
disinfectant and the waste material. A shredder can also pass
relatively large items unscathed. If a sizing screen were used
downstream of a shredder at this point, it would quickly clog.
The output of the granulating stage is preferably fully wetted by
the disinfectant solution, and it has a smaller granular particle
size than the output of the shredding stage. The granulating stage
is also designed with a sizing screen interacting with the
specially designed blades to insure that the waste material is
repeatedly cut and separated. The blades tend to press outwardly on
the waste material in addition to cutting it, partially forcing the
waste material through the screen. The blades also drag waste
material cross the surface of the screen, thereby dispersing any
tightly packed clumps of the material. This insures that the waste
material does not clog the screen, and that it is reduced to the
appropriate size to achieve thorough contact with the disinfectant,
thereby allowing use of the desired low concentration of
chemical.
The outlet from the fragmenting chamber incorporates the
aforementioned screen functioning in cooperation with the
granulating blades. The screen is sized to allow a selected smaller
granular particle size waste to fall through the screen into a
disinfectant reactor chamber below, while retaining any waste which
has not been sufficiently granulated in the granulating stage.
Waste which is retained by the screen is scooped up by the
granulating blades rotating against the screen and returned to the
associated cutting surfaces for additional particle size reduction
until the waste is sufficiently small to pass through the screen.
Up to this point substantially all of the work to convey the waste
through the above-recited stages is performed by gravity.
The granulating stage is followed by the disinfecting stage. The
disinfecting stage comprises a disinfectant reactor chamber
preferably integral with an auger. The auger has two ends; a liquid
medium collection tank and inlet port are at one end of the auger
and a disinfected solid waste discharge port is at the other end.
The auger is inclined upwardly to convey the waste from the inlet
port to the disinfected solid waste discharge port. The length of
the auger wherein the disinfection reaction occurs constitutes the
disinfectant reactor chamber. The disinfection reaction is
preferably completed by the time the waste reaches a point about
two-thirds up the auger incline. The controlled rate at which the
auger screw carries the waste up the incline to the discharge port
enables a sufficient residence time for disinfection of the
waste.
The disinfecting stage is combined with a dewatering stage. The
dewatering stage comprises a conical flow restriction immediately
prior to the discharge port. Although some of the liquid medium is
removed from the waste by gravity at the lower end of the auger,
the bulk of the liquid medium is removed from the waste by
compressing the mash through the flow restriction. The final exit
from the auger is positioned at or slightly beyond the point at
which the conical flow restriction begins. The conical flow
restriction is constructed with a critical restriction angle best
suited to dewater the waste being treated. When these features are
combined with the proper granulating of the waste, and when the
waste is free of long strips, a high degree of dewatering will
result without resulting in clogging of the discharge path. The
auger is also sloped at a critical angle best suited to assist in
dewatering while avoiding clogging. The combination of the pressure
rise in the mash resulting from the conical restriction, and the
pressure rise resulting from the angle of the auger, yield a
compression of the waste material which achieves the maximum
dewatering efficiency without resulting in clogging.
The liquid medium driven from the waste mash exits the auger
through perforations or a screen in the housing surrounding the
auger, and the liquid is collected and passed to the heated
disinfectant mixing tank for recycling to the wetting stage. The
screen in the auger housing is shaped to conform to the radial
edges of the auger, so that as the auger turns it continually
scrapes compacted waste material from the screen. This prevents
clogging of the screen. The liquid to be recycled is passed through
a cyclone separator designed to remove heavy fines prior to return
of the liquid to the jets. The heavy fines can be periodically
dumped from the cyclone separator onto the waste material in the
auger.
In operation, process control for the present system is provided by
regulating the disinfectant concentration in the system as a
function of the liquid medium temperature. Temperature is in turn a
function of liquid medium flow, and heater and auger operating
parameters. It is apparent that the above-described system
satisfies the present objective of providing an infectious waste
treatment apparatus which contacts precise amounts of a
disinfectant with an infectious waste to disinfect the waste while
simultaneously fragmenting the waste to reduce its bulk volume. It
is further apparent that the system provides an infectious waste
disposal apparatus which is relatively easy and comparatively
cost-effective to implement and operate.
The novel features of this invention, as well as the invention
itself, both as to its structure and its operation, will be best
understood from the accompanying drawings, taken in conjunction
with the accompanying description, in which similar reference
characters refer to similar parts, and in which:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of the multi-stage waste treatment
apparatus of the present invention;
FIG. 2 is a view of the shredding stage of the apparatus of FIG. 1,
along line 2--2;
FIG. 3 is a schematic view of the apparatus of FIG. 1;
FIG. 4 is a section view of an alternate embodiment of the waste
outlet flow restriction;
FIG. 5 is a schematic of a control unit for the apparatus of the
present invention;
FIG. 6 is a generalized curve for the functional relation between
disinfectant solution temperature and disinfectant
concentration;
FIG. 7 is a schematic view of the granulating stage of the
apparatus of FIG. 1;
FIG. 8 is an enlarged view of a portion of FIG. 7, showing the
relationship between the blades in the granulating stage; and
FIG. 9 is a schematic view of the auger of FIG. 1, with a cyclone
separator .
DESCRIPTION OF PREFERRED EMBODIMENTS
Referring initially to FIGS. 1 and 3, the infectious waste
treatment system of the present invention is generally designated
10. System 10 comprises a plurality of treatment stages including
an inlet stage 12, a shredding stage 14, a wetting stage 16, a
granulating stage 18, a disinfecting stage 22, and a dewatering
stage 24 which define a continuous flowpath for the waste. The
terms "disinfect" and "decontaminate" are used synonymously herein
and refer to the destruction of a substantial portion of infectious
constituents within the infectious waste sufficient to render the
waste substantially noninfectious.
Inlet stage 12 comprises an opening 28 at or near the top of a
fragmenting chamber 30 which houses stages 14, 16, and 18. Inlet
stage 12 opens down into shredding stage 14 at the upper level of
fragmenting chamber 30. As shown in FIG. 2, shredding stage 14
comprises multiple pairs of rotatable shredding blades 31a,b,
32a,b, 33a,b. Blades 31a, 32a, 33a, are mounted on shaft 34 and
blades 31b, 32b, 33b are mounted on shaft 35 such that blade 31b is
rotatably fitted between blades 31a, 32a and so on for all the
blades as shown. Each rotatable shredding blade is a disk 36 having
a plurality of hook-shaped teeth 37 about the periphery 38 of disk
36. Stationary shredding blades such as 39a, 39b are fixed to
chamber walls 40 spaced appropriately from rotatable blades 31a,
32a, 33a and 31b, 32b, 33b to channel waste into the rotatable
blades, to reduce the waste particle size to a first selected
particle size, and to prevent waste from accumulating in shredding
stage 14. The output of waste material from the shredding stage 14
will include some long strips of relatively soft material. Shafts
34, 35 are positioned horizontally and parallel to one another, and
the rotatable blades rotate in vertical planes which are
substantially parallel to the vertical flowpath of the waste.
Shredding action is provided by rotating shaft 34 in the opposite
direction from shaft 35.
Referring to FIG. 3, a wetting stage 16 is provided immediately
downstream from shredding stage 14. Wetting stage 16 comprises a
plurality of liquid disinfectant jets 42a,b,c,d which are mounted
in the wall 40 of chamber 30 around the periphery of the waste
flowpath and adjacent the bottom side of the shredding blades. A
liquid medium feed line is connected to each jet. Thus, as shown in
FIG. 1, liquid medium feed lines 46a,b,c,d are connected to jets
42a,b,c,d respectively. Feed lines 46a,b,c, are also connected to a
recycle pump 48 across a liquid distribution manifold 50. Pump 48
receives liquid medium from a recycle line 54 connected to a liquid
medium collection tank 56 of a recycle stage 26. The manifold 50 is
also fed by the output of a disinfectant mixing tank 156.
Precursors, or constituents, of the desired disinfectant are fed
into the mixing tank 156 through inlet lines 158. The disinfectant
thus formulated is then immediately pumped from the mixing tank 156
into the manifold 50 by pump 160 and disinfectant line 162. This
arrangement is particularly useful where the desired disinfectant
is very volatile, such as chlorine dioxide. The liquid precursors
can be sodium chlorite and citric acid. Recycled disinfectant can
be fed back into the mixing tank through a recycle line 144.
Recycle line 144 can be fed by a cyclone separator as will be
discussed later. The mixing tank 156 can be maintained at a
selected elevated temperature by heaters in the tank.
If it is desired to remove metals from the waste stream after
shredding and wetting, a metal segregating stage 58 may be provided
immediately after stages 14, 16. Metal segregating stage 58
comprises a magnet 60 which is mounted in the wall 40 of chamber
30. Magnet 60 contacts the waste as it falls toward granulating
stage 18 to segregate the metals therefrom. Access is provided in
wall 40 to enable periodic removal of metals from magnet 60.
Granulating stage 18 is positioned at the lower level of
fragmenting chamber 30 and comprises a plurality of rotatable
granulating blades 62a,b,c,d,e and stationary granulating blades
64a,b. Referring to FIGS. 3, 7, and 8, the rotatable blades
62a,b,c,d,e are mounted on a rotating shaft 66 which in turn is
rotatably mounted on chamber wall 40. The rotatable blades have a
vertical plane of rotation which is substantially parallel to the
vertical flowpath of the waste. The rotatable blades 62a,b,c,d,e
are rotatable past stationary granulating blades 64a, 64b, each of
which is fixably mounted on opposite sides of chamber wall 40
adjacent rotatable blades 62a,b,c,d,e. As rotatable blades
62a,b,c,d,e rotate, they periodically pass stationary blades 64a,b
to form transient cutting surfaces. FIG. 3 shows rotatable blade
62e meeting stationary blade 64b to form transient cutting surface
68. The rotatable and stationary granulating blades are all
preferably formed with rectangular cross-sections as shown, so that
each blade has four potential cutting edges. FIG. 8 shows the
relationship between the cutting edges in more detail. Each blade
can be removed and rotated to expose a new cutting edge, until all
four edges on each blade have been dulled. As each rotatable blade
passes each stationary blade, the clearance between the cutting
edges is sufficiently small to granulate the material by a cutting
action. The material is then continually passed through the blades
until sufficient cuts have been made to reduce the waste material
to a selected second, smaller particle size. In addition, all
strips of waste material are granulated by this cutting action.
A screening action is accomplished immediately beneath the
granulating blades in granulating stage 18. This comprises a screen
70 stretched cross-sectionally across conduit 72 which connects
fragmenting chamber 30 and auger 74. Screen 70 has a mesh size
which allows particles at or below a given particle size to pass
through while preventing particles having a larger particle size
than the given particle size from passing through. The movement of
the rotatable blades imparts an outward radial motion to the waste
material which partially imbeds the waste material in the screen
70. Screen 70 preferably has a 1/2 inch mesh size although other
mesh sizes are within the purview of the skilled artisan. Screen 70
is positioned to cooperate with the rotatable granulating blades
62a,b,c,d,e of granulating stage 18. As the rotatable blades
rotate, they periodically pass screen 70 to scoop waste retained on
screen 70. FIG. 7 shows rotatable blade 62d meeting screen 70 to
return waste retained by screen 70 to cutting surface 68.
Disinfecting stage 22 comprises a disinfectant reaction chamber 76
which is integral with auger 74. Auger 74 is inclined upward away
from auger inlet 78 to enable precise control of the waste
residence time in reaction chamber 76 and to facilitate dewatering
as described hereafter. The inclination angle of auger 74 is
defined as .phi.. For a waste stream composed mostly of soft
material, such as medical waste, .phi. is selected between about
10.degree. and 20.degree. and preferably about 15.degree.. This
aids in dewatering, without promoting clogging. A lesser angle
results in less dewatering capability, while a greater angle
appreciably increases the tendency to clog. Reaction chamber 76 is
sufficiently sized to hold the throughput of system 10 for a
residence time which enables disinfection of the waste before
discharge from system 10. Auger 74 has a screw 80 extending axially
the entire length of auger 74 which is rotatably mounted therein to
carry waste from auger inlet 78 to a waste solid discharge port 86
at the upper end of auger 74.
Dewatering stage 24 is likewise integral with auger 74 and
comprises a conical flow restriction 90 at solid disinfected waste
discharge port 86. A portion of liquid medium exits auger 74 under
gravity through port 82 to collection tank 56 in fluid
communication with port 82. A perforated plate 88 is provided at
port 82 having a plurality of perforations 89, each significantly
smaller than the mesh size of screen 70, and preferably about 1/8
inch, to prevent substantial quantities of waste from exiting auger
74 thereat. However, the primary function of port 82 is to enable
fluid intrusion into auger 74 as will be shown.
The conical flow restriction 90 imposes a pressure on the waste
material which compacts the material and removes the bulk of liquid
medium from the waste before it exits system 10. The auger screw 80
terminates slightly beyond the entrance to the conical restriction
90. In one embodiment the constriction is a conical nozzle 90
having a fixed opening at the end of waste discharge port 86. The
angle of the conical restriction 90 is selected according to the
content of the waste stream. For a waste stream composed mostly of
soft material, such as medical waste, the angle is between 15 and
20 degrees, and preferably about 18 degrees. This ensures
sufficient compaction of waste material to achieve dewatering,
without clogging the flow path. A lesser angle would significantly
detract from the dewatering ability, while a greater angle would
significantly increase the tendency to clog. In another embodiment,
FIG. 4 shows an adjustable nozzle comprising a pair of doors 92a,
92b, the lower door having a pneumatically biased hinge 93 to
render the size of opening 91 pressure responsive. In any case, the
restriction applies a compacting force to the disinfected waste
before the waste exits the system 10.
Liquid medium driven from the disinfected waste by the compacting
force exits auger 74 through perforations 94 in auger housing
screen 96. Perforations 94 are sized small enough to restrict the
solid waste from the liquid stream. A sleeve 98 around screen 96 at
perforations 94 channels the liquid medium into a recycle line 100
which is in fluid communication with the mixing tank 156 through
recycle inlet line 144. Before being recycled to the mixing tank
156, as shown in FIG. 9, the liquid is passed through a cyclone
separator 140 by means of a pump (not shown). The liquid cycles
through the separator 140 to exit into the recycle inlet line 144,
after the separation of heavy fines 146 which fall to the bottom of
the separator 140. Periodically, a valve 150 is opened to flush the
heavy fines 146 out the outlet 148 of the separator 140 and back
onto the waste material on the auger 74.
Collection tank 56 has two chambers 104, 106 in fluid communication
with one another, but separated by a weir 108. Port 82 of auger 74
is submerged in primary chamber 104. Secondary chamber 106 receives
the overflow of primary chamber 104 and has a recycle outlet port
110 connected to recycle line 54. Heater elements 112,114 are
submerged in primary and secondary chambers 104, 106 respectively
for heating the liquid medium as necessary. The collection tank 56
and the mixing tank 156 can be combined as one tank without
departing from the spirit of the invention.
FIG. 5 is a schematic for process control of system 10 which is
provided by automated control unit 120 in electrical communication
with auger 74, heaters 112, 114, recycle pump 48 and door 92b.
Control unit 120 accordingly regulates the speed of auger screw 80,
the heat output of heaters 112,114, the liquid medium recycle rate
of pump 48 and the compaction force applied by door 92b to the
waste at solid waste discharge port 86. These parameters are
regulated in response to the primary input parameters to unit 120
which are the ClO.sub.2 concentration and the temperature of the
liquid medium in tank 56. ClO.sub.2 concentration data is provided
to unit 120 by means of a conventional air stripper 122 in tank 56
and ClO.sub.2 gas analyzer 124. Temperature data is provided to
unit 120 from a conventional thermometer 126.
It is understood that although waste treatment system 10 has been
described above in a specific sequence of multiple stages, certain
stages may be omitted or reordered within the scope of the present
invention as is apparent to one skilled in the art. As such, the
present invention is not limited to the above-recited sequence of
stages.
METHOD OF OPERATION
With cross-reference to the drawings, operation of system 10 in a
continuous mode may be seen. System 10 is particularly suited to
the treatment of infectious wastes generated by hospitals and other
medical facilities. Such wastes are primarily solid wastes
consisting of plastic, paper, fabric, glass, and metal and embody a
broad range of medical items including syringes, bottles, tubes,
dressings, and the like. "Waste treatment" as the term is used
herein constitutes fragmenting of the waste to a relatively small
granular particle size and disinfecting the waste to render it
substantially innocuous and suitable for ordinary landfilling.
The infectious waste is fed through inlet opening 28 into system 10
in any form. In a preferred embodiment, however, the waste is
stored in a sealed compartmentalized plastic bag 128 which is then
fed through opening 28 into system 10 in its entirety. Waste bag
128 has a primary compartment 130 containing the infectious waste,
and the bag can have other prefilled and sealed compartments 132,
134 containing disinfectant chemicals or other process additives,
if called for, which are introduced into system 10 via inlet
opening 28. Additives may include dyes, defoamers, or
surfactants.
The waste is inserted through inlet opening 28 into the top of
fragmenting chamber 30 by an operator. The waste drops under the
force of gravity from opening 28 down into opposingly rotating
shredding blades 31a,b, 32a,b, 33a,b of shredding stage 14. The
shredding blades destroy waste bag 128, spilling the waste and
additives into chamber 30 where they are commingled to form a waste
mixture. The shredding blades also break up the frangible waste to
a small particle size. Wetting stage 16 operates simultaneously
with stage 14, whereby the disinfectant jets wet the waste mixture
with a stream of a liquid disinfectant. The liquid disinfectant is
pumped to the jets from lines 54 and 62 connected to liquid medium
collection tank 56 and mixing tank 156. With efficient operation of
dewatering stage 24, the bulk of liquid medium in system 10 is
recycled. The liquid disinfectant may be within a temperature range
between about 0.degree. C. and 100.degree. C. and preferably
between about 5.degree. C. and 70.degree. C. The liquid medium has
more preferably been preheated above ambient temperature to an
elevated temperature of at least about 40.degree. C. and most
preferably at least about 50.degree. C.
The liquid disinfectant uniformly contacts the falling waste
mixture to form a wet mash. The mash falls through metal
segregating stage 58 where metals are removed and continues falling
down into granulating stage 18 where the rotating blades and the
stationary blades break up the already small particle size
frangible waste into yet a smaller granular particle size which is
preferably slightly less than 1/4 inch. The granulating blades also
fragment any fibrous material which has not been previously
fragmented by the shredding blades to about the same smaller
granular particle size as the frangible material. The granulating
blades also more fully mix the mash. Thus, the solids in the
resulting mash of granulating stage 18 are preferably fully wetted
by the disinfectant solution and the bulk of the solids preferably
have a smaller granular particle size which is slightly less than
about 1/4 inch. The liquids content of the mash is typically on the
order of about 60% by weight.
Upon exiting granulating stage 18, the mash drops onto screen 70
which functions in cooperation with the granulating stage 18 to
allow the smaller granular particle size waste to fall through it
into disinfectant reactor chamber 76 while retaining any waste in
granulating stage 18 which has not been sufficiently fragmented.
Waste which is retained by screen 70 is scooped up by the rotating
granulating blades rotating against screen 70, and returned to
cutting surface 68 for additional particle size reduction until it
is sufficiently small to pass through screen 70.
Inlet port 78 receives the waste mash from screening stage 20 and
directs the mash to reactor chamber 76 integral with auger 74. The
disinfectant solution collected in primary chamber 104 contacts the
mash at lower end 84 of auger 74. Auger screw 80 turns continuously
to withdraw the mash from lower end 84 at angle .phi. up the auger
incline to solid waste discharge port 86 at a controlled rate which
allows a sufficient residence time of the mash in reactor chamber
76. A sufficient residence time is typically on the order of less
than about 5 minutes and preferably on the order of about 3
minutes. Auger screw 80 also maintains perforated screen 96 free of
waste so that the liquid medium may exit the auger to be recycled.
The disinfected and dewatered waste exiting system 10 typically has
a liquids content of about 20% by weight in contrast to a liquids
content in the mash of about 60% by weight.
The bulk waste volume of the exit waste is on the order of about
15% of the inlet waste. Most of the liquid medium is removed from
the waste as the result of compaction caused by fixed nozzle 90 or
pressure responsive nozzle 92a,b positioned at waste discharge port
86. The liquid medium exits auger 74 through perforations 94 and is
collected in tank 156 for recycling to wetting stage 16 via line
162. Alternatively, collection can be in tank 56. The dual-chamber
weir arrangement of tank 56 enables collection of fines in primary
chamber 104 for periodic removal.
Process control for system 10 is provided by control unit 120. The
decontamination level, i.e., level of kill, attainable in system 10
is a function of several interrelated operating parameters
including liquid medium flow parameters and auger and heater
operating parameters as shown in FIG. 5. Nevertheless, as is shown
below, an operational model of system 10 can be developed as a
function of a limited number of key parameters, which are level of
kill, disinfectant concentration and temperature.
Accordingly, process control can be effected by selecting a desired
level of kill, i.e., target kill, and adjusting the disinfectant
concentration and disinfectant solution temperature as a function
of the operating parameters to meet the preselected target kill.
For example, a target kill of 6 decades (10.sup.6 organisms/ml) is
achieved within about three minutes for a typical infectious
medical waste using a chlorine dioxide solution at a concentration
of 30 ppm and a temperature of 50.degree. C. In practice, however,
the process is controlled by adjusting only temperature while
monitoring variations in the disinfectant concentration as a
baseline for temperature adjustment. Temperature is selected as the
independent variable and disinfectant concentration as the
dependent variable for the practical reason that the ability to
independently adjust disinfectant concentration is somewhat limited
when a fixed amount of precursor is employed, while it is
relatively easy to adjust solution temperature via heaters 112,
114.
The operational model of system 10 recognizes the functional
relationship between solution temperature and concentration of the
disinfectant, chlorine dioxide, at a given level of kill n. The
model is represented by the equation:
wherein
[ClO.sub.2 ]=chlorine dioxide concentration,
T=temperature, and
a.sub.n, k.sub.n =empirically determined constants for
kill.sub.n.
FIG. 6 generally depicts the shape of the curve for equation (1).
Each point on the curve defines values of [ClO.sub.2 ] and T at
which kill.sub.n can be achieved. Accordingly, process control is
more specifically implemented by preselecting the target kill,
empirically determining the model constants at the target kill to
define a curve, and adjusting the actual values of [ClO.sub.2 ] and
T to lie on the target kill curve.
FIG. 6 shows a typical start-up scenario for system 10. The
treatment solution is initially at point A which is inside the
required curve for the target kill. Since it is desirable to
operate on the curve, automated process control 120 consequently
raises the temperature of the solution in tanks 56, 156 toward
point B which corresponds to the same chlorine dioxide
concentration as point A, but at a higher temperature. Raising the
temperature of the solution, however, increases the rate of
chlorine dioxide formation, thereby increasing the chlorine dioxide
concentration of the solution to a value designated by C on the
vertical axis. Thus, as point B is approached, control unit 120
calculates that the required temperature on the curve has fallen.
The dashed line shows the iterative equilibration procedure
followed by control unit 120 whereby an operating point designated
by D is ultimately attained. Operation is preferably maintained
along or above the locus of points making up the curve which
includes point D.
Chlorine dioxide concentration in tank 56 is continuously monitored
by means of air stripper 122 and gas analyzer 124 to enable control
unit 120 to determine whether the requirements of the disinfectant
solution have changed. For example, if a relatively "dirty" waste
is fed to system 10, the amount of ClO.sub.2 consumed increases,
reducing the ClO.sub.2 concentration in the solution. Accordingly,
control unit 120 must iteratively increase the temperature of the
solution in the manner recited above to return operation of system
10 to the curve. If a relatively "clean" waste is fed to system 10,
the ClO.sub.2 concentration increases, correspondingly reducing the
temperature requirement. Thus, control unit 120 decreases the
temperature of the solution. It is preferable to preselect a target
kill exceeding a minimum acceptable level of kill so that adequate
decontamination of the waste is achieved even when operation falls
somewhat below the curve. It has generally been found that within
the presently prescribed temperature range a minimum ClO.sub.2
concentration in the treatment solution to achieve an acceptable
level of kill is about 10 ppm up to the required concentration and
preferably about 12 ppm up to the required concentration.
As noted in the preferred embodiment above, starting quantities of
the chlorite salt and acid are fixed. As such, they are preferably
provided in stoichiometric excess of quantities necessary to
produce the required chlorine dioxide concentrations shown on the
curve of FIG. 6. Thus, adequate concentrations of liquid precursors
will be available in solution for chlorine dioxide production
despite the fact that, in most cases, some of the precursors do not
react, and the additional fact that a significant fraction of the
chlorine dioxide is consumed by reaction with the infectious waste
constituents or diffuses out of solution. By way of example, a
typical relative starting concentration of precursors, solvent and
waste which will provide a desired chlorine dioxide concentration,
is on the order of 4.6 g/l sodium chlorite/3.3 g/l citric acid/12
kg of solid waste.
While the particular Multi-Stage Infectious Waste Treatment System
as herein shown and disclosed in detail is fully capable of
obtaining the objects and providing the advantages herein before
stated, it is to be understood that it is merely illustrative of
the presently preferred embodiments of the invention and that no
limitations are intended to the details of construction or design
herein shown other than as described in the appended claims.
* * * * *